Possibility to use kv filter against a previously groked variable

Let's write a filter section that looks like this

    grok {
        type => 'demo'
        pattern => "%{URIPATH:mypath}%{URIPARAM:myparams}"
    }
    kv {
        type => 'demo'
        fields => ???
        field_splut => "&?"
    }

As of today if I want to use the kv filter against the `myparams` variable I got from grokking I can't. kv needs to received on of the top-level fields (ie. @source, @tags, @fields, etc...)

This pull request aims to make the preceding possible, with the following snippet

    grok {
        type => 'demo'
        pattern => "%{URIPATH:mypath}%{URIPARAM:myparams}"
    }
    kv {
        type => 'demo'
        fields => ["@fields.myparams"]
        field_split => "&?"
    }

I hope this make sense,